Do a lot of reading. A *lot*. When you read, you pick up writing techniques from the author(s), and also perhaps metaphors and similes that you might use. It also gives you a better sense for writing, and as a matter of fact (at least, this is what I've noticed), my writing has made me a *much* better reader than I was before.

Use all your senses, not just hearing and eyesight, when you're bringing your settings to life. Give a sense of what things feel like, smell like, and taste like as you write. This isn't just set dressing; it can be used to come up with some new and fresh observations and metaphors.

This one may seem like an "oh, duh!" but here it is: NEVER write without using a spell-checker! Spelling mistakes often drag the reader right back into the real-world, when you want them immersed in your story!